O’Bannon, ordered 28 June 1940, was fitted with the original high, rounded bridge carried over from the Sims class. Like the other two 2,100-ton Fletcher-class ships completed in June 1942, Nicholas and Fletcher, she initially carried a 1.1-inch anti-aircraft cannon between the “53” and “54” 5-inch gunhouses.

The 1.1-inch gun tended to overheat and jam in service, however, and was replaced by a twin 40mm Bofors in the same location atop the aft deckhouse, which O’Bannon carried through the remainder of the war. In 1944, O’Bannon was further modified to carry two more of these twin mounts abreast the after stack and an additional two forward, below the bridge, in common with 157 of 166 then-active Fletchers: